A reciprocating engine requires a crankshaft for converting the reciprocating motion of pistons in cylinders to rotational motion so as to extract power. Crankshafts are generally categorized into two classes: the type manufactured by die forging and the type manufactured by casting. Especially when high strength and high stiffness are required, the firstly mentioned die forged crankshafts, which are excellent in these properties, are often employed.
FIG. 1 is a schematic side view of an example of a common crankshaft. A crankshaft 1 shown in FIG. 1 is designed to be mounted in a 4-cylinder engine and includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr, a flange Fl, and eight crank arms A1 to A8 (hereinafter also referred to simply as “arms”) that connect the journals J1 to J5 and the crank pins P1 to P4 to each other. The crankshaft 1 is configured such that all of the eight crank arms A1 to A8 are formed integrally with counterweights W1 to W8 (hereinafter also referred to as “weights”), respectively, and is referred to as a 4-cylinder 8-counterweight crankshaft.
Hereinafter, when the journals J1 to J5, the crank pins P1 to P4, the crank arms A1 to A8, and the counterweights W1 to W8 are each collectively referred to, the reference character “J” is used for the journals, “P” for the crank pins, “A” for the crank arms, and “W” for the counterweights. A crank pin P and a pair of crank arms A (including the counterweights W) which connect with the crank pin P are also collectively referred to as a “throw”.
The journals J, the front part Fr, and the flange Fl are arranged coaxially with the center of rotation of the crankshaft 1. The crank pins P are arranged at positions eccentric with respect to the center of rotation of the crankshaft 1 by half the distance of the piston stroke. The journals J are supported by the engine block by means of sliding bearings and serve as the central rotational axis. The big end of a connecting rod (hereinafter referred to as “conrod”) is coupled to the crank pin P by means of a sliding bearing, and a piston is coupled to the small end of the conrod.
In an engine, fuel explodes within cylinders. The combustion pressure generated by the explosion causes reciprocating motion of the pistons, which is converted into rotational motion of the crankshaft 1. In this regard, the combustion pressure acts on the crank pins P of the crankshaft 1 via the conrod and is transmitted to the journals J via the respective crank arms A connecting to the crank pins P. In this process, the crankshaft 1 rotates while repetitively undergoing elastic deformation.
The bearings that support the journals of the crankshaft are supplied with lubricating oil. In response to the elastic deformation of the crankshaft, the oil film pressure and the oil film thickness in the bearings vary in correlation with the bearing load and the journal center orbit. Furthermore, depending on the surface roughness of the journals and the surface roughness of the bearing metal in the bearings, not only the oil film pressure but also local metal-to-metal contact occurs. Ensuring a sufficient oil film thickness is important in order to prevent seizure of the bearings due to lack of lubrication and to prevent local metal-to-metal contact, thus affecting the fuel economy performance.
In addition, the elastic deformation accompanied with the rotation of the crankshaft and the movements of the center orbit of the journals within the clearances of the bearings cause an offset of the center of rotation, and therefore affect the engine vibration (mount vibration). Furthermore, the vibration propagates through the vehicle body and thus affects the noise in the vehicle and the ride quality.
In order to improve such engine performance properties, there is a need for a crankshaft having high stiffness with the ability to resist deformation. In addition, there is a need for weight reduction of the crankshaft.
A crankshaft is subjected to loads due to pressure in cylinders (combustion pressure in cylinders) and centrifugal force of rotation. In order to impart deformation resistance to the loads, an attempt is made to improve the torsional rigidity and the flexural rigidity of the crankshaft. In designing a crankshaft, the main specifications such as the journal diameter, the crank pin diameter, and the piston stroke are firstly determined. The region to be designed after determination of the main specifications is the shape of the crank arms. Thus, the design of the crank arm shape for increasing both the torsional rigidity and the flexural rigidity is an important requirement. Strictly speaking, as described above, the crank arms mean the oval portions connecting the journals and the pins to each other and do not include the portions serving as counterweights.
In the meantime, a crankshaft needs to have a mass distribution that ensures static balance and dynamic balance so as to be able to rotate kinematically smoothly as a rotating body. Accordingly, an important requirement is to adjust the mass of the counterweight region with respect to the mass of the crank arm region determined by the requirements for the flexural rigidity and torsional rigidity, in view of weight reduction while ensuring the static balance and dynamic balance.
For the static balance, the adjustment is made so that when the mass moment (the “mass” multiplied by the “radius of the center of mass”) of the crank arm region and the counterweight region are summed, the result is zero. For the dynamic balance, the adjustment is made so that, when, for each region, the product of the axial distance from the reference point to the center of mass multiplied by the mass moment (the “mass” multiplied by the “radius of the center of mass” multiplied by the “axial distance”) is determined using a point on the rotation axis of the crankshaft as the reference and the products are summed, the result is zero.
Furthermore, the balance ratio is adjusted for balancing against the load of combustion pressure within one throw (a region of the crankshaft corresponding to one cylinder). The balance ratio is defined as a ratio of the mass moment of the counterweight region to the mass moment of the crank arm region including the crank pin (and also including part of the conrod, strictly speaking) in the crankshaft, and this balance ratio is adjusted to fall within a certain range.
There is a trade-off between an increase in stiffness of the crank arm of a crankshaft and a reduction in weight thereof, but heretofore various techniques relating to the crank arm shape have been proposed in an attempt to meet both needs. Such conventional techniques include the following.
Japanese Patent No. 4998233 (Patent Literature 1) discloses a crank arm having intensively greatly depressed recess grooves in the crank pin-side surface of the crank arm and the journal-side surface thereof, on a straight line connecting the axis of the journal to the axis of the crank pin (hereinafter also referred to as a “crank arm centerline”). The crank arm disclosed in Patent Literature 1 is intended to achieve a reduction in weight and an increase in stiffness. The recess groove in the journal-side surface contributes to a reduction in weight by virtue of the reduced mass, and moreover, the thick region around the recess groove contributes to an increase in torsional rigidity. However, in reality, an increase in flexural rigidity cannot be substantially expected because of the intensively greatly depressed recess grooves on the crank arm centerline.
Japanese Translation of PCT International Application Publication No. 2004-538429 (Patent Literature 2), Japanese Translation of PCT International Application Publication No. 2004-538430 (Patent Literature 3), Japanese Patent Application Publication No. 2012-7726 (Patent Literature 4), and Japanese Patent Application Publication No. 2010-230027 (Patent Literature 5) each disclose a crank arm having a greatly and deeply depressed hollow portion in the journal-side surface of the crank arm, on the crank arm centerline. The crank arms disclosed in Patent Literatures 2 to 5 are also intended to achieve a reduction in weight and an increase in torsional rigidity. However, in reality, the flexural rigidity is reduced because of the greatly and deeply depressed hollow portion on the crank arm centerline.